What is the gravity of the situation? Using geodesy to characterize unique volcanic systems

Investigating the subsurface geological structures of active volcanic systems and their evolution in time is one of the main pursuits of modern volcanology. Many geophysical methods are routinely used for the monitoring and exploration of volcanoes, but none are superior to gravimetry to characterize subsurface mass variations despite its challenges. In this work, two unique volcanic systems were analyzed through the lens of geodetic methods, allowing for a better understanding of the dynamic processes occurring in the subsurface. Pre-eruptive, co-eruptive, and post eruptive gravity changes and ground deformation at Sierra Negra Volcano (Galápagos Islands, Ecuador) are some of the largest detected in any volcanic system around the world, with persistent meter-scale pre-eruptive inflation and hundreds of μGal of gravity increase occurring before eruptive activity. Through the monitoring and modelling of gravity changes, in tandem with deformation signals and their gravitational effects, the total magmatic recharge before the 2018 eruption of Sierra Negra was constrained. The post-eruptive rate of magmatic recharge was estimated at 2 × 10^10 kg/year, which if maintained, would recover the previous mass gained in between eruptive periods in the next 17 to 18 years. The Garibaldi Volcanic Belt (GVB), British Columbia, is distinguished from the rest of the Cascade Volcanic Arc due to the geochemical signature of the northern centres' eruptive products. Bouguer gravity anomaly mapping was performed to investigate the subsurface structure of these volcanoes; inverse modelling of these gravity anomalies revealed large negative density contrasts (∼ -280 kg/m^3) located beneath the northern edifices of the GVB. The subsurface structures of the Mount Meager Volcanic Complex, a target for geothermal exploration, were also modelled to reveal two zones related to the active hydrothermal system, and a negative density anomaly to the north of the massif that may be related to either deep, supercritical hydrothermal fluids, or to a pluton-like shallow chamber. This work paves the way for future joint modelling of geophysical datasets informed by geological data, which will further the understanding of the anomalous density contrasts detected in the GVB and their link to its unique geochemical signature.